Synchronous full-bridge oscillator

ABSTRACT

An electronic high frequency induction heater driver, for a variable spray fuel injection system, uses a zero-voltage switching oscillator that utilizes a full H-bridge topology wherein the semiconductor switches are synchronized within the bridge for function. The induction heater driver, upon receipt of a turn-on signal, multiplies a supply voltage through a self-oscillating series resonance, wherein one component of the tank resonator circuit comprises an induction heater coil magnetically coupled to an appropriate loss component so that fuel inside a fuel component is heated to a desired temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional of, and claims priority to theApr. 22, 2011, filing date of, U.S. provisional patent application Ser.No. 61/478,350, entitled Synchronous Full-Bridge Power Oscillator, theentire content of which is incorporated herein by reference.

And this application is related to the following U.S. non-provisionalpatent applications filed on the same day as this application:

Synchronous Full-Bridge Power Oscillator with Leg Inductors, invented byPerry Czimmek, and identified by Attorney Docket Number 2011P00689US01;

Synchronized Array Bridge Power Oscillator, invented by Perry Czimmekand Mike Hornby, and identified by Attorney Docket Number2011P00691US01;

Synchronized Array Power Oscillator with Leg Inductors, invented byPerry Czimmek and Mike Hornby, and identified by Attorney Docket Number2011P00692US01;

Variable Spray Injector with Nucleate Boiling Heat Exchanger, inventedby Perry Czimmek and Hamid Sayar, and identified by Attorney DocketNumber 2011P00693US01; and

Adaptive Current Limit Oscillator Starter, invented by Perry Czimmek,and identified by Attorney Docket Number 2011P00694US01.

BACKGROUND

Embodiments of the invention relate generally to heated tip fuelinjectors, and more particularly, to controlling and driving aninduction-heated fuel injector.

There is a continued need for improving the emissions quality ofinternal combustion engines. At the same time, there is pressure tominimize engine crank times and time from key-on to drive-away, whilemaintaining maximum fuel economy. Those pressures apply to enginesfueled with alternative fuels such as ethanol as well as to those fueledwith gasoline.

During cold temperature engine start, the conventional spark ignitioninternal combustion engine is characterized by high hydrocarbonemissions and poor fuel ignition and combustibility. Unless the engineis already at a high temperature after stop and hot-soak, the crank timemay be excessive, or the engine may not start at all. At higher speedsand loads, the operating temperature increases and fuel atomization andmixing improve.

During an actual engine cold start, the enrichment necessary toaccomplish the start leaves an off-stoichiometric fueling thatmaterializes as high tail-pipe hydrocarbon emissions. The worstemissions are during the first few minutes of engine operation, afterwhich the catalyst and engine approach operating temperature. Regardingethanol fueled vehicles, as the ethanol percentage fraction of the fuelincreases to 100%, the ability to cold start becomes increasinglydiminished, leading some manufacturers to include a dual fuel system inwhich engine start is fueled with conventional gasoline and enginerunning is fueled with the ethanol grade. Such systems are expensive andredundant.

Another solution to cold start emissions and starting difficulty at lowtemperature is to pre-heat the fuel to a temperature where the fuelvaporizes quickly, or vaporizes immediately (“flash boils”), whenreleased to manifold or atmospheric pressure. Pre-heating the fuelreplicates a hot engine as far as fuel state is considered.

A number of pre-heating methods have been proposed, most of whichinvolve preheating in a fuel injector. Fuel injectors are widely usedfor metering fuel into the intake manifold or cylinders of automotiveengines. Fuel injectors typically comprise a housing containing a volumeof pressurized fuel, a fuel inlet portion, a nozzle portion containing aneedle valve, and an electromechanical actuator such as anelectromagnetic solenoid, a piezoelectric actuator or another mechanismfor actuating the needle valve. When the needle valve is actuated, thepressurized fuel sprays out through an orifice in the valve seat andinto the engine.

One technique that has been used in preheating fuel is to inductivelyheat metallic elements comprising the fuel injector with a time-varyingmagnetic field. Exemplary fuel injectors having induction heating aredisclosed in U.S. Pat. No. 7,677,468, U.S. Patent Application No's:20070235569, 20070235086, 20070221874, 20070221761 and 20070221747, thecontents of which are hereby incorporated by reference herein in theirentirety. The energy is converted to heat inside a component suitable ingeometry and material to be heated by the hysteretic and eddy-currentlosses that are induced by the time-varying magnetic field.

The inductive fuel heater is useful not only in solving theabove-described problems associated with gasoline systems, but is alsouseful in pre-heating ethanol grade fuels to accomplish successfulstarting without a redundant gasoline fuel system.

Because the induction heating technique uses a time-varying magneticfield, the system includes electronics for providing an appropriate highfrequency alternating current to an induction coil in the fuel injector.

Conventional induction heating is accomplished with hard-switching ofpower, or switching when both voltage and current are non-zero in theswitching device. Typically, switching is done at a frequency near thenatural resonant frequency of a resonator, or tank circuit. Theresonator includes an inductor and capacitor that are selected andoptimized to resonate at a frequency suitable to maximize energycoupling into the heated component.

The natural resonant frequency of a tank circuit is fr=1/(2π√{squareroot over (LC)}), where L is the circuit inductance and Cis the circuitcapacitance. The peak voltage at resonance is limited by the energylosses of the inductor and capacitor, or decreased quality factor, Q, ofthe circuit. Hard-switching can be accomplished with what are calledhalf-bridge or full-bridge circuits, comprising a pair or two pairs ofsemiconductor switches, respectively. Hard-switching of power results inthe negative consequences of switching noise, and high amplitude currentpulses at resonant frequency from the voltage supply, or harmonicsthereof. Also, hard switching dissipates power during the linear turn-onand turn-off period when the switching device is neither fullyconducting nor fully insulating. The higher the frequency of ahard-switched circuit, the greater the switching losses.

The preferred heater circuit therefore provides a method of driving aheated fuel injector wherein switching is done at the lowest possibleinterrupted power. This heater circuit was disclosed in U.S. Pat. No.7,628,340, Title: Constant Current Zero-Voltage Switching InductionHeater Driver for Variable Spray Injection. Ideally, energy should bereplenished to the tank circuit when either the voltage or the currentin the switching device is zero. It is known that the electromagneticnoise is lower during zero-voltage or zero-current switching, and islowest during zero-voltage switching, this is the method of U.S. Pat.No. 7,628,340. It is also known that the switching device dissipates theleast power under zero switching. That ideal switching point occurstwice per cycle when the sine wave crosses zero and reverses polarity;i.e., when the sine wave crosses zero in a first direction from positiveto negative, and when the sine wave crosses zero in a second directionfrom negative to positive.

It is preferable to reduce the size of inductive components and in somecases, eliminate the impedance-matching transformer, while maintainingthe minimum necessary connections to the inductive heater coil on theinjector. The difficulties of this will be detailed in thisspecification in subsequent text. Embodiments of the invention continueto provide for the elimination of the hard-switching and its negativeconsequences, replace it with zero-voltage switching, and further applythis method in a full-bridge topology while advantageously eliminatingthe impedance matching transformer and overcoming the difficulties ofalternative solutions.

Referring to FIG. 3, the description disclosed in U.S. Pat. No.7,628,340 includes a center-tapped impedance matching transformer thatthose skilled in the art will be familiar with as to the additional costthis topology might present.

Referring to FIG. 4, elimination of the impedance matching transformeris possible and practical, but with the result that the induction heatercoil uses a center-tap to satisfy the push-pull oscillator topology.This center-tap adds an additional conductor and pin to the inductivelyheated fuel injector.

Referring to FIG. 5, the further elimination and change of the topologyof the push-pull oscillator follows from the FIG. 4 topology, however,it uses two constant-current inductors of a relatively much higherinductance compared to the induction heater coil to allow sufficientcurrent to flow through the impedance of the heater coil, otherwise themajority would flow only through the power switches. In order tomaintain sufficient heater current, this tends to result in a smallinductance of the heater coil rather than a reduction ofconstant-current inductance. This reduction of heater coil inductancetends to the point that parasitic inductance of the wire harness andconnections may overwhelm that of the heater coil. Additionally, inorder to obtain a small heater coil inductance, the number of coil turnsis reduced for a given magnetic mass available, possibly sacrificingAmpere-Turns per Hertz and thereby detrimentally affecting the abilityto perform induction heating.

BRIEF SUMMARY

Embodiments of the invention eliminate the impedance matchingtransformer. Further, embodiments of the invention eliminate thecenter-tap of the induction heating coil such that only two conductorsare used for transmission of power. Additionally, embodiments of theinvention force current sharing through the induction heating coil whileallowing for flexibility and suitable inductance and ampere-turns of theinduction heater coil.

One or more embodiments use two pairs of complimentary pairs of powerswitching transistors in a full-bridge, or H-bridge, configuration. Adeviation from a full-bridge driver is that the bridge is fed from aconstant-current source inductor, and the load section of theconventional full bridge is replaced with a resonant tank circuit.Further deviation from a conventional full-bridge is anoscillator-synchronous inherent zero-switching topology that drives thegates of the complimentary pairs of transistors in alternating sequenceof diagonal pairs.

Additionally, the tank-replenishment current passes through theinduction heater coil, and the conventional center-tap current feed inthe original push-pull oscillator, wherein only half the coil at a givencycle received the tank-replenishment current, is eliminated.Eliminating the center-tap in this way provides a minimum wire solutionto driving the induction heater coil without an impedance matchingtransformer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified electrical schematic diagram showing asynchronous bridge oscillator with source inductor and without atransformer and without a center-tap in accordance with embodiments ofthe invention.

FIG. 2 is a simplified electrical schematic diagram showing asynchronous bridge oscillator with sink inductor and without atransformer and without a center-tap in accordance with embodiments ofthe invention.

FIG. 3 is a simplified schematic diagram depicting the prior art with acenter-tapped impedance matching transformer in a push-pull oscillatortopology.

FIG. 4 is a simplified schematic diagram depicting the prior art with acenter-tapped induction heater coil and elimination of the impedancematching transformer in a push-pull oscillator topology.

FIG. 5 is a simplified schematic diagram depicting an alternativesolution to the elimination of the impedance matching transformer andelimination of heater coil center-tap.

FIG. 6 is an alternative schematic layout of FIG. 1, with an addedlow-side control switch as a demonstration of an alternative embodimentof the invention.

DETAILED DESCRIPTION

Ideally, energy should be replenished to the tank circuit when eitherthe voltage or the current in the switching device is zero. Theelectromagnetic noise is lower during zero-voltage or zero-currentswitching and is lowest during zero-voltage switching. The switchingdevice dissipates the least power under zero switching. That idealswitching point occurs twice per cycle when the sine wave crosses zeroand reverses polarity; i.e., when the sine wave crosses zero in a firstdirection from positive to negative, and when the sine wave crosses zeroin a second direction from negative to positive.

Embodiments of the invention eliminate hard-switching and its negativeconsequences, and replace it with zero-voltage switching in afull-bridge configuration. The integrated functions of the synchronousfull-bridge power oscillator heater driver of the invention will beexplained with reference to FIG. 1, which is a simplified representationof a circuit in accordance with embodiments of the invention with manyof the basic components not shown for clarity. Specific or generalvalues, ratings, additions, inclusion or exclusion of components are notintended to affect the scope of the invention.

L1 may be located inside a fuel injector. L1 is an induction heater coilthat provides ampere-turns for induction heating a suitablefuel-injector component.

A synchronous full-bridge power oscillator in accordance withembodiments of the invention may include R1, R2, D1, D2, Q1, Q2, Q3, Q4,L2, C1 and L1. Q1 and Q2 are enhancement-type N-MOSFET (N-channelMetal-Oxide-Semiconductor Field-Effect Transistor) switches thatalternatively connect tank resonator, C1 and L1, circuit to ground and,when each is turned on in the respective state, enables current to flowthrough induction heater coil and ground. Q3 and Q4 are enhancement-typeP-MOSFET (P-channel Metal-Oxide-Semiconductor Field-Effect Transistor)switches that alternatively connect tank resonator, C1 and L1, circuitto the voltage supply sourced through L2, and with Q1 and Q2 in theappropriate state, enable current to flow through induction heater coil.The voltage supply may be a power supply, or in the case of a vehicle, abattery or an alternator, and is a source of potential energy toreplenish energy lost in the oscillator.

C1 and L1 are the tank resonator capacitor and tank resonator inductor,respectively, of a resonant tank circuit. The resonant frequency of thetank circuit is fr=1/(2π√{square root over (LC)}), where L is the heatercoil inductance L1 and C is the capacitance of tank capacitor C1. Thepeak voltage in the tank circuit is set by V_(out)=π*V_(in) where V_(in)is the supply voltage. The current level in the tank circuit isdetermined from the energy balance of

${\frac{1}{2}{LI}^{2}} = {\frac{1}{2}{{CV}^{2}.}}$

The zero-switching power oscillator circuit is self-starting inoscillation, but may be forced into oscillation by selectivelysequencing the switching of Q1-Q4 in a full-reversing H-bridge strategy.The complimentary pairs, or here, the pairs of transistors that areflowing current between the MOSFET ‘drain’ and ‘source’ at the same timeare Q3 and Q2 or Q4 and Q1. It is not desirable to have Q1 flowingcurrent when Q3 flows current, and likewise, it is not desirable to haveQ2 flowing current when Q4 flows current. When Q3 is flowing current,current passes through the induction heater coil and then through Q2 toground. When Q4 is flowing current, current passes through the inductionheater coil in the reverse direction as when Q3 was flowing current, andthen through Q1 to ground, this is ‘full-reversal’ of current.

A MOSFET is a device that has a threshold for an amount of Coulombcharge into the gate, which is drain-source current-dependent.Satisfying the charge threshold enhances the device into an ‘on’ state.First and second gate resistors R1, R2 supply the gate charging currentto first and second legs of the H-bridge. R1 supplies current to gatesof Q1 and Q3, R2 supplies current to the gates of Q2 and Q4,respectively, and R1, R2 limit the current flowing into first and secondgate diodes D1, D2, respectively. Q3 and Q4, P-MOSFET conduct betweendrain and source when source is more positive than gate. Q1 and Q2,N-MOSFET conduct between drain and source when source is more negativethan gate.

The loading caused by the resistive and hysteretic loss of the heatedcomponent reflects back as a loss in the resonant tank circuit. Thatloss is replenished by current flowing from a current source inductor L2to the top bridge transistors, Q3 and Q4. FIG. 2 shows a variation withL2 as a sink inductor from the bottom bridge transistors, Q1 and Q2,those skilled in the art will understand this alternative embodiment asfunctionally interchangeable overall. Depending on the state of reversalof the H-bridge in which the current flows, the current will flow eitherthrough Q3 or Q4 and then through induction heater coil L1. Referringagain to FIG. 1, L2 supplies current to the tank circuit from the energystored in its magnetic field. That energy is replenished from the supplyvoltage as a current that constantly flows into L2 during operation ofthe synchronous full-bridge power oscillator. L2 also provides transientseparation of tank circuit from voltage source.

If current is flowing through Q3, as determined by the polarity of thesine wave half-cycle at that time, then the conduction to ground from Q2drain-to-source is pulling charge out of the gate of Q3 and Q1 throughforward biased D1. Q1 is also now not conducting and does not pull thegate charge out of Q4 and Q2 to ground through D2. Meanwhile R1 drawscurrent from the supply voltage. But the IR drop across R1 cannot chargethe gate of Q3 and Q1 with the gate shunted to ground by conductionthrough Q2.

When the sine wave crosses zero, then Q3 becomes reverse biased andconducts through the internal intrinsic diode to reverse-bias D1. D1stops conducting current away from the Q3 and Q1 gate, and R1 can chargethe gate of Q3 and Q1, which stops conduction in Q3 and startsconduction in Q1 to begin conducting current for the continuing sinehalf-cycle. Q1 also pulls the gate charge out of Q2 and Q4 to groundthrough D2 and holds Q2 in a non-conducting state, which continues toallow R1 to enhance Q1. And Q4 conducts.

That process repeats as the sine wave alternates polarity, crossing zeroin a first direction from negative to positive, and then in a seconddirection from positive to negative. This generates full-reversal ofcurrent in L1, the induction heater coil. Current continues to bereplenished in the tank circuit from L2. An IGBT (Insulated Gate BipolarTransistor) device can replace the N-MOSFET in this embodiment if theintrinsic diode of the N-MOSFET is represented by the addition of anexternal diode across the drain and source of the IGBT.

The foregoing detailed description is to be understood as being in everyrespect illustrative and exemplary, but not restrictive, and the scopeof the invention disclosed herein is not to be determined from thedescription of the invention, but rather from the claims as interpretedaccording to the full breadth permitted by the patent laws. For example,while the synchronous full-bridge power oscillator of the invention isdescribed herein driving an induction heater coil for the heater in aninternal combustion engine fuel injector, the driver may be used todrive other induction heaters in other applications. It is to beunderstood that the embodiments shown and described herein are onlyillustrative of the principles of the present invention and that variousmodifications may be implemented by those skilled in the art withoutdeparting from the scope and spirit of the invention.

1. A power oscillator for an electronic induction heater driver, thepower oscillator comprising: an H-bridge circuit topology comprisinghigh-side and low-side semiconductor switches; a resonant tank circuitelectrically connected between legs of the H-bridge in the topologicallocation of a conventional H-bridge load, wherein the resonant tankcircuit includes at least one induction heater coil; at least one energyreplenishment inductor electrically in series with the H-bridge; whereinthe H-bridge switch timing is determined by a frequency of the resonanttank circuit.
 2. The power oscillator of claim 1, wherein the energyreplenishment inductor sources current to the H-bridge from a voltagesource.
 3. The power oscillator of claim 1, wherein the energyreplenishment inductor sinks current from the H-bridge to an absolutepotential less than the voltage source.
 4. The power oscillator of claim1, wherein the H-bridge switch timing is accomplished through rectifierdiodes sinking charge from one leg of the H-bridge to the opposite legof the H-bridge.
 5. The power oscillator of claim 1, wherein theH-bridge switch timing is accomplished through resistors sourcing chargefrom the voltage supply.
 6. The power oscillator of claim 1, wherein aninductance of the energy replenishment inductor is greater than aninductance of the induction heater coil.
 7. The power oscillator ofclaim 1, wherein an inductance of the energy replenishment inductor ismore than twice an inductance value of the induction heater coil.